This invention relates to an improvement in a nonlinear capacitor for generating high-voltage pulses, used, for example, in the contactless starter of a gas-discharge lamp.
Ferroelectric ceramic capacitors (hereinafter referred to as FEC capacitors) made mainly from barium titanate have found various applications as circuit elements. Since their nonlinear voltage charge characteristics enable them to function as high-speed electronic switches, FEC capacitors are also used nowadays as the switching elements in the contactless starter of gas discharge lamps, as disclosed, for example, in Japanese Patent Publication No. 48-28726.
An FEC capacitor utilized as a switching element in a contactless starter for a gas discharge lamp is generally constructed as follows: as shown in FIG. 1, both sides of a sintered body 1 of barium titanate (BaTiO.sub.3) powder are coated with electrode layers 2a and 2b, respectively. These electrode layers 2a and 2b are connected to lead wires 3a and 3b, respectively. Then the entire surface of the body is coated with an insulating material 4, such as epoxy resin.
A contactless starter eqipped with FEC capacitor thus constructed has been put into practical use as a contactless starter for a fluorescent lamp. Recently. attempts have been made to use it in the built-in starter of a high-intensity gas-discharge lamp (hereinafter referred to as an HID lamp) such as a high pressure sodium lamp. As yet, however, such a built-in contactless starter for an HID lamp utilizing FEC capacitors has not been put into practical use.
This may be attributable to the following fact: there are special characteristics required of an FEC capacitor that is to be used in a built-in starter for an HID lamp. First, it must offer a sufficient thermal resistance not to deteriorate or suffer a change in properties when exposed to a high temperature gas within the lamp. Second, it must be able to generate pulses of sufficient amplitude and energy to permit easy and reliable starting of an HID lamp that needs a high starting voltage.
An FEC capacitor of the construction shown in FIG. 1 cannot satisfy the above requirements, so that it cannot be used in a starter for an HID lamp.
In view of this, the inventor of the present invention previously proposed an FEC capacitor construction in Japanese Patent Publication No. 62-60803, which is shown in FIG. 2. In this capacitor, electrode layers 12a and 12b are formed on both sides of a ferroelectric ceramic substrate 11 which is made mainly from barium titanate or a similar material. The entire surface of the substrate 11 is then coated with a mineral glass 13 containing lead oxide and boric oxide as its main constituents, with only the central portions of the electrode layers 12a and 12b left uncoated, and to these central portions 12a and 12b are affixed lead terminals 15a and 15b by means of an electrically conductive adhesive 14a, 14b.
In the above-described capacitor construction, silver paste is employed for forming the electrode layers 12a and 12b. For the electrically conductive adhesive 14a, 14b for affixing the lead terminals 15a and 15b to the electrode layers 12a and 12b, a paste mixture is used which consists of silver powder and a low-melting-point glass powder containing lead oxide (PbO) and boric oxide (B.sub.2 O.sub.3) as its main constituents.
A mineral glass which offers thermal resistance as the coating material prevents the capacitor thus constructed from deteriorating in a high-temperature gas. It has been found, however, that there is still room for improvement in the amplitude and energy of the pulse.
The difficulty of generating pulses of sufficient amplitude and energy may be attributable to the following circumstances: when an FEC capacitor is applied to a contactless starter to generate high-voltage pulses in cooperation with a choke coil type ballast, the FEC capacitor is subjected to mechanical oscillation due to electrostriction caused by the high-voltage pulses generated. In the case of a disc type FEC capacitor, the oscillation takes place in the radial and thickness directions, as shown in FIGS. 3A and 3B. The greater the amplitude of this oscillation without being restrained by external factors, the higher the voltage of the high-voltage pulses generated. In the case of an FEC nonlinear capacitor of the above-described construction, such mechanical oscillations due to electrostriction may be restrained because of the fact that the entire surface of the element is coated with a mineral glass, which will interfere with improvement in the amplitude and energy of the pulses.
It has also been found that in a nonlinear capacitor of the above-described construction, the electrically conductive adhesive 14a, 14b is dispersed through the electrode layers 12a and 12b over the grain boundary of the ferroelectric ceramic substrate 11, exerting an undesirable influence on the capacitor characteristics. In the first place, this causes a general decrease in the value of the high-voltage pulses generated. This phenomenon is attributable to the intrusion of impurities into the ferroelectric ceramic substrate. Furthermore, the stresses due to the mechanical oscillation of the ferroelectric ceramic substrate caused by electrostriction during high-voltage pulse generation may cause a fracture in the ferroelectric ceramic substrate itself at the grain boundaries where such an adhesive dispersion has taken place.